In order to improve the recovery of hydrocarbons from oil and gas wells, the subterranean formations surrounding such wells can be hydraulically fractured. Hydraulic fracturing is used to create small cracks in subsurface formations to allow oil or gas to move toward the well. Formations are fractured by introducing specially engineered fluids at high pressure and high flow rates into the formations through the wellbores. Hydraulic fractures typically extend away from the wellbore 250 to 750 feet in two opposing directions according to the natural stresses within the formation.
The fracture fluids are preferably loaded with proppants, which are usually particles of hard material such as sand. The proppant collects inside the fracture to permanently “prop” open the new cracks or pores in the formation. The proppant creates a plane of high-permeability sand through which production fluids can flow to the wellbore. The fracturing fluids are preferably of high viscosity, and therefore capable of carrying effective volumes of proppant material.
Recently, there has been an effort to monitor hydraulic fracturing and produce maps that illustrate where the fractures occur and the extent of the fractures. Current hydraulic fracture monitoring comprises methods of processing seismic event locations by mapping seismic arrival times and polarization information into three-dimensional space through the use of modeled travel times and/or ray paths. Travel time look-up tables may be generated by modeling for a given velocity model. A typical mapping method is commonly known as the “Non-Linear Event Location” method. Non-linear event location has been used historically to locate macro seismic events such as earthquakes, and is described, for example, at http://geoazur.unice.fr/PERSO/lomax/nlloc/. This and other equivalent methods are referred to herein as non-linear event location methods.
The non-linear event location methods involve the selection and time picking of discreet seismic arrivals for each of multiple seismic detectors and mapping to locate the source of seismic energy. However, to successfully and accurately locate the seismic event, the discrete time picks for each seismic detector need to correspond to the same arrival of either a “P” or “S” wave and be measuring an arrival originating from the same microseismic or seismic event. During a fracture operation, many hundreds of microseismic events may be generated in a short period of time. Current techniques employed in the industry require considerable human intervention to quality control the time picking results. It can often take weeks from the time of recording and detecting the microseismic events to produce accurate maps of the event locations. Even so, the result, which requires human interaction and interpretation, can lead to multiple and non-reproducible solutions.
Therefore, current methods of real-time monitoring and modeling of fracture growth are typically based on pumping and pressure data, which provides very limited information concerning the geometry of fracture growth. There is a need for microseismic event detection and location that can be implemented in real time and enable an operator to adjust hydraulic fracture parameters during the fracture job.